Energy stores for storing and releasing electric energy are of great significance for many mobile applications, for example. While the storage capacity of modern energy stores for storing electric energy is sufficient for the operation of relatively small devices, such as mobile phones, portable computers etc., energy stores for storing electric energy for larger applications, such as electrically driven motor vehicles, are still subject to shortcomings which stand in the way of their commercially successful use. The storage capacity of the batteries used, in particular, does not yet meet the aimed-for standards. Although lithium-ion batteries, for example, achieve good results for use in mobile phones or computers for instance, they are of only limited suitability for applications requiring a large amount of energy, such as electrically operated motor vehicles. Here, the storage capacity of lithium ion batteries represents a limiting factor, e.g. for the range of an electric motor vehicle.
In the motor vehicle sector, in particular, there are furthermore known systems in which the energy required for driving is stored in the form of hydrogen. By means of a fuel cell, the hydrogen is then converted into electric current, by means of which the motor can be driven. However, the construction of a refueling network for hydrogen is necessary for such a technology, and this makes the introduction of this technology expensive, especially in view of the high safety requirements of refueling stations because of the risk of explosions.
More recently, batteries in which the electric energy is stored in the form of an oxidation state of a metal have furthermore also been considered. The construction of a battery of this kind corresponds approximately to that of a fuel cell with a solid electrolyte. The electrolyte is arranged between two electrodes, one of which is an air electrode composed of a material which cleaves the atmospheric oxygen and conducts the oxygen ions formed in the process to the electrolyte. The electrolyte is likewise produced from a material which can conduct oxygen ions. Arranged on the opposite side thereof from the air electrode is the second electrode, which is composed of a metal or metal oxide to be oxidized and reduced. The battery is discharged by a process in which the metal is oxidized by means of oxygen ions from the atmospheric oxygen, and is charged by a process in which the metal is reduced upon application of a voltage, releasing oxygen ions, wherein the oxygen ions then migrate through the electrolyte to the air electrode, from where they are released to the surroundings as molecular oxygen. This process is illustrated schematically in FIG. 1, in which the upper half illustrates the discharge process and the lower half illustrates the charging process. In this figure, reference numeral 101 denotes the battery, reference numeral 103 denotes the air electrode, reference numeral 105 denotes the metal or metal oxide, reference numeral 107 denotes the electrolyte, reference numeral 109 denotes a load supplied with current as the battery is discharged, and reference numeral 111 denotes a power source used in the charging of the battery.
The aim is to improve the power density of the batteries described in order to implement the system with a minimum size and maximum economy. It is important here to prevent unwanted oxidation of the metal due to penetration of air into the battery. Air penetration in the region of the metal electrode leads to power losses extending as far as complete failure of the battery, depending on the quantity of air which has penetrated.
The electrolytes used in the batteries exhibit highly selective oxygen ion conduction but require relatively high operating temperatures, typically 600° C. or above. At such temperatures, sealing the battery against air penetration requires a lot of design effort and a high outlay on materials since many sealing materials cannot be used, owing to the high temperatures.
Cited documents U.S. Pat. No. 5,492,777, DE102009057702A1 and U.S. Pat. No. 4,204,033 describe energy stores having solid electrolytes and iron as the oxidizable material for energy storage. Moreover, steam/hydrogen gas are used as a redox pair in the energy stores. DE 102009057702A furthermore describes that the partial pressure of the hydrogen gas is 1 bar and the partial pressure of the steam is 10−3 bar.